I know the question has been asked but looking for any updates on how other have dealt with the need to auto calculate fields in WAB. Attribute rules come with a lot of limitations and are not plausible at this time (i.e., ArcMap not supported, branch versioning required which prohibits archiving, etc.). We use SQL Server so I can write a database trigger. I have done this before but I hate doing it for a host of reasons. I also use python scripts that run in batch for calculations that can be done periodically (e.g., nightly). However, my current need is for real time calculation upon feature edit. It amazes me that this is not provided by means of a calculate expression in the Smart Editor.
Calling all smart colleagues! Can you help a brother out?